The invention relates to a method for the manufacture of a substrate provided with a chromium VI-free and a cobalt-free passivation, the substrate provided with such a passivation as well as a device for carrying out the method and an aqueous, acidic composition for passivating which is suitable for use in the aforementioned method.
The passivation of metallic substrates has proven its worth; however, the components of the passivation solutions have turned out to be a risk from a health and an environmental protection point of view. Thus chromium (VI) compounds frequently can no longer be used and also elements, such as cobalt and nickel, frequently are no longer desirable.
A chromium (VI)-free and cobalt-free composition for passivating is disclosed in U.S. Pat. No. 4,578,122. Nitrate ions and chromium (III) compounds in an aqueous, acidic solution are used, additionally activating metal ions, such as iron, aluminum, lanthanum or cerium ions being added. The ratio of nitrate ions on the one hand to chromium (III) ions and activating metal ions on the other should not be less than 4:1.
In the future, the addition of the activating metal ions, indicated in U.S. Pat. No. 4,578,122, either is no longer desirable for reasons of health or the environment or the use of metal ions is expensive.
Published patent application DE 3 213 384 discloses a first acidic and a second alkaline passivation which is free of chromium VI and cobalt. However, this two-step passivation is not yet optimized with regard to corrosion protection.